Tornado Sirens, an Old Technology, Still Play a Vital Role

For residents of tornado-prone regions like Oklahoma, Texas and the southern Plains, the piercing wail of outdoor sirens in May — the most active month — is a warning to seek shelter because a funnel cloud has been spotted or is on its way.

In an age of text alerts, social media and smartphones, you might think sirens had outlived their usefulness in alerting the public to approaching severe weather.

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A tornado south of Wynnewood, Okla., on May 9. Tornado warnings used to be banned because researchers believed they would promote panic.CreditJosh Edelson/Agence France-Presse

But officials and enthusiasts believe they continue to serve a vital role in public safety.

When were tornado sirens first used?

Logan Shelts, the lead moderator of Air Raid Sirens, an online forum of educators and enthusiasts, said tornado sirens were an outgrowth of the Cold War, when they were installed to alert communities in case of a nuclear attack or an air raid.

Sirens were first used to warn of tornadoes in 1970. That was a big improvement from alerts in times gone by.

How loud are sirens?

Generally, they can be heard within one mile, but their reach varies by their size and the topography, atmospheric conditions and seasons, said Aaron Wolking, a national sales manager with Sentry Siren in Canon City, Colo.

Larger sirens have a two-mile radius. How do federal authorities determine how loud a siren should be? An effective range is calculated by measuring its decibel output at 100 feet and then applying a formula developed by federal authorities that accounts for how sound waves fade over a given distance, Mr. Wolking said. The company’s largest siren can put out 130 decibels.

Why do sirens make that noise?

The loudness and pitch of sirens “exploit basic principles of human audition to signal danger,” Elizabeth Hellmuth Margulis, a professor of music and the director of the Music Cognition Lab at the University of Arkansas, said in an email.

In addition, sound is “especially good at carrying emotional resonance,” so the tone you heard as a child during drills will prompt “deep and immediate” feelings later in life, she said.

Most sirens emit sounds between 400 and 600 hertz, which researchers have found is the best range to get people’s attention, said Scott Yarberry, the chief operating officer and executive vice president of Sentry Siren.

Are there different types of sirens?

There are mechanical sirens:

There are electronic sirens:

Sirens can put out an “alert” tone — a three-minute, uninterrupted sound — or an “attack” tone that modulates and sounds like a classic air-raid siren.

Some sirens are stationary, and others rotate; they also come in a variety of shapes and sizes. Some are dual tone, meaning they produce two pitches at once.

Does it still make sense to have sirens?

For out-of-the-way places, such as golf courses, lakes and hiking trails, where cellphone service might be spotty or nonexistent, sirens are an “important redundancy” to alert the public, said Bill Bunting, the chief of forecast operations at the Storm Prediction Center in Norman, Okla.

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Most sirens emit sounds between 400 and 600 hertz, which researchers have found is the best range to get people’s attention.CreditRogelio V. Solis/Associated Press

Mr. Shelts said social media can be ineffective in delivering warnings because there is no true management or vetting of what gets posted. Incomplete, inaccurate or outdated information could be shared.

Sirens also hold up well: One unit that Sentry Siren installed in Maryland has been in continuous service since the 1920s, Mr. Yarberry said.

Do the sirens have other uses?

Fire departments use them to summon volunteers, and some communities use them to warn of the threat of large hail or powerful winds.

Yeshiva Toras Emes Kamenitz in Brooklyn uses Sentry Siren’s largest siren, a model 40V2T with a two-mile radius, to sound the start of the Sabbath, Mr. Yarberry said.

Correction:May 23, 2016

An earlier version of this article misstated how the estimated range of a warning siren is calculated. The effective range is calculated by measuring the siren’s decibel output at 100 feet and then applying a formula developed by federal authorities that accounts for how sound waves fade over a given distance, not 70 decibels from 100 feet away.